From Garde on the forum:
it may be useful to have a third type of reminder to remind reviewers a day or two BEFORE the review is due. I am increasingly receiving emails like that (when I am a reviewer somewhere) and I think it is a really polite way of reminding reviewers that their review is due soon (i.e. before they have missed the dealine and thus give them the chance to submit in time)
(See http://pkp.sfu.ca/support/forum/viewtopic.php?f=3&t=7519&p=29076#p29076)